[NTLUG:Discuss] Re: LVM Span and data loss -- RAID-0, 1, 5, FRAID v. Intelligent RAID, etc...

[Bryan J. Smith]

On Sun, 2004-11-28 at 22:16, Jay Urish wrote:
> Raid 0 is mirroring...

Just FYI (me being anal), I believe RAID-1 is mirroring.

RAID-0 is typically considered striping (although some refer to it as
spanning too) and non-redundant.

> What you need to do is find a raid controller with built in
> BIOPS like the one of the 3ware units..

Again, just FYI (me being anal), _every_ RAID card (and even non-
RAID ATA, SCSI, etc... cards) has 16-bit Int10h BIOS services so
it can boot.  The "FRAID" (free RAID) cards add in a bit of setup
logic.

Where the "FRAID" cards differ with either the 3Ware ASIC+SRAM
non-blocking I/O or traditional uC+DRAM buffering RAID cards is
the on-board intelligence.  I.e., once the OS loads, the FRAID
cards have 100% of their logic in the OS driver and use the main
CPU.  The ASIC+SRAM or uC+DRAM approaches have an on-board
intelligence with full embedded OS, SRAM/cache or DRAM/buffer to
handle queuing or cache/buffering operations, etc...

They also only require a minimal block driver which is typically
GPL because they do not expose any IP -- unlike the FRAID driver
which is typically licensed logic (although there is a generic,
"ataraid" GPL driver in the Linux kernel that can and is used with
the "hptraid", "pdcraid" and "silraid" per-FRAID card drivers, but
it is wholly unsupported by the vendors and may differ from their
on-disc organization).

For more on the 4 types of ATA RAID options:  
- Typical OS Logical Volume Management (LVM)
- Cheap Free RAID (FRAID) Cards
- Traditional uC+DRAM Intelligent RAID
- 3Ware non-blocking I/O ASIC+SRAM Intelligent RAID

See the article "Dissecting ATA RAID Options" in the 2004 April
issue of Sys Admin:  
  http://www.samag.com/articles/2004/0404/  

Unfortunately it is not a featured web article, so you need to
have a subscription or have bought it at the store.  Subscribers
get _all_ past issues on CD, just FYI.  ;->

> Then snag another 250gb drive and set up raid5

RAID-5 is _not_ necessarily better.  It is more efficient disk-wise,
and works like an N-1 (where N = number of disks) RAID-0 when it
comes to _reads_, but RAID-3, 4 and 5 are _much_slower_ when it comes
to writes than RAID-0, RAID-0+1 or even RAID-1 in some cases.

"Efficiency" is discussed in the "Dissecting ATA RAID Options" article
including a _detailed_ table on read v. write v. disk cost.  I guess
I could repost that table if someone wanted it.

FYI, the 3Ware Escalade 7000 and 8000 series with their small 1-4MB of
SRAM are _not_ ideal for RAID-5.  The small 0 wait state SRAM is perfect
for RAID-0, 1 and 0+1 operations, but for lots of random RAID-5 writes,
it can overflow and stall write operations.

Something like either the 3Ware Escalade 9000 series with 128MB of
SDRAM, or some of the new Intel IO331/332 (XScale) superscalar
microcontroller based ATA RAID controllers with 128MB of SDRAM would be
far more ideal for RAID-5.

In reality, I've found 4+ disc RAID-0+1 on a 3Ware Escalade 8506 series
to be the most ideal in price/performance.

Sometimes a combination is good as well.  E.g., using 3Ware Escalade
7506 and 8506 series products with 8 or 12 channels, I'll use 2-disc
RAID-1 or 4-disc RAID-0+1 for the "system" (especially swap, /tmp and
/var), and then 6-8 disc RAID-5 for the "data" partitions (especially
read-only databases or home directories).


-- 
Bryan J. Smith                                    b.j.smith at ieee.org 
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